Voltage modulator circuit to control light emission for non-invasive timing measurements

ABSTRACT

A non-invasive method and apparatus accurately measures the time difference between two signal edges by optically detecting the emission from a ‘beacon device’ that is modulated as a function of time difference. Through the use of this modulation it is possible to perform timing measurement accurately. Embodiments of a voltage modulator circuit modulate timing information into emission intensity. The method and system of the present invention can be used in applications such as clock skew and pulse width measurements.

FIELD OF THE INVENTION

[0001] Embodiments of the present invention relate to a voltagemodulator circuit to control light emission for non-invasive timingmeasurements.

BACKGROUND OF THE INVENTION

[0002] Recent microprocessor designs use a flip-chip assembly to improvepower distribution and achieve higher operating frequencies. Debugprobing of such devices relies on what is known as Laser Voltage Probing(LVP). However, LVP technology cannot accurately measure edge delays formulti-GHz frequencies and the laser invasiveness is increasing withsmaller transistor geometries.

[0003] To overcome these problems, methods to translate signal edgetiming information into light emission that can be accurately measuredusing a Time-Resolved Emission (TRE) or InfraRed-Emission Microscope(IREM) have been proposed. These methods are based on the phenomenonthat hot electrons in a saturated NMOS transistor (or beacon device)emit infrared radiation both under static bias and switching condition.See T. Eiles, et. al., “Optical Probing of Flip-Chip PackagedMicroprocessors”, ISSCC Digest of Technical Papers, pp. 220-221,February 2000, and L. T. Hoe, et. al., “Characterization and Applicationof Highly Sensitive Infra-Red Emission Microscopy for MicroprocessorBackside Failure Analysis”, Proceedings of the 7^(th) IPFA, pp. 108-112,1999. Thus, as indicated in FIG. 1, infra-red light is emitted from anNMOS transistor 10 when in saturation, i.e., Vds>Vgs−Vt.

[0004] J. C. Tsang et al., in “Picosecond hot electron emission fromsubmicron complementary metal oxide semiconductor circuits,” Appl. Phys.Lett., p.889-891, February 1997 describes using a commonly available,very low noise optical detector such as mercury cadmium telluridedetector array, which has good sensitivity in the range of 0.9-1.45 μm,one can measure the emission intensity (I_(emission)) accurately. Theuse of light emission for time-dependent analysis is described by DanKnebel et al. in “Diagnosis and Characterization of Timing-relatedDefects by Time-dependent Light Emission”, International TestConference, p. 733-739, August 1998. This paper describes clock skewanalysis as one of many potential applications. In addition, it suggeststhe use of a phase-detector circuit (PFC) to modulate the duration oflight pulse as a function of skew.

[0005] Thus, as shown in FIG. 2, in the prior art, a Phase-FrequencyComparator (PFC) 11 is used to focus the mode of operation on oneparticular edge (i.e. rising edge) for which a timing delay Δt is to bemeasured. The PFC is coupled to a saturated NMOS transistor (or beacondevice)13 which emits infrared radiation. The radiation is then detectedby a photon detector 15, which may be a TRE or IREM as noted above. Theresulting measured pulse, has a width representing Δt.

[0006] However, we have found that this method is limited by a ‘deadbandregion’ where, if the skew is less than the rise/fall time of the clockunder test, it will go undetected. A need, therefore, exists for amethod and apparatus which overcomes this limitation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a schematic diagram of a beacon transistor of the priorart which is used in embodiments of the method and apparatus of thepresent invention.

[0008]FIG. 2 is a block diagram of a prior art arrangement for measuringtiming information through the use of light emission from asemiconductor.

[0009]FIG. 3 is graph plotting intensity vs. gate voltage for a beacontransistor such as that of FIG. 1.

[0010]FIG. 4 is a block diagram of an embodiment of the presentinvention for measuring timing information through the use of lightemission from a semiconductor.

[0011]FIG. 5 is a circuit diagram of a modulator according to anembodiment of the present invention.

[0012]FIG. 6 is a timing diagram for the embodiment of FIG. 5.

[0013]FIGS. 7a-7 c are graphs showing the relationship of clock skew vs.resistance, current and voltage respectively, for an embodiment of acircuit according to FIG. 5.

[0014]FIG. 8 is showing timing skew vs. intensity and gate voltage andintensity vs. gate voltage for embodiments of the method and apparatusof the present invention.

DETAILED DESCRIPTION

[0015] Embodiments of methods and systems for measuring timinginformation through the use of light emission from a semiconductor aredescribed. In the following description, for purposes of explanation,numerous specific details are set forth to provide a thoroughunderstanding of the present invention. It will be appreciated, however,by one skilled in the art, that the present invention may be practicedwithout these specific details. In other instances, structures anddevices are shown in block diagram form. Furthermore, one skilled in theart can readily appreciate that the specific sequence in which methodsare presented and performed are illustrative and it is contemplated thatthe sequences can be varied and still remain within the spirit and scopeof the present invention.

[0016] Embodiments of the present invention provide a novel,non-invasive method and system to accurately measure the time differencebetween two signal edges. This is accomplished by optically detectingthe emission from a ‘beacon device’ that is modulated as a function oftime difference. Through the use of this modulation it is possible toperform timing measurement accurately. The system for doing thisincludes embodiments of a voltage modulator circuit to modulate timinginformation into emission intensity. The method and system of thepresent invention can be used in applications such as clock skew andpulse width measurements would benefit from the new technique.

[0017] Experimental measurements have shown that the light emission froma saturated NMOS device has an exponential relationship with the gatevoltage, Vgs as shown in FIG. 3. Illustrated is a curve 21 for an NMOStransistor and a curve 23 for a PMOS transistor. The present inventionemploys this dependency to accurately translate the timing differencebetween two signal edges into a Vgs voltage level, thus modulating theI_(emission). Specifically, the timing difference is first converted toa voltage level Vgs. The voltage level Vgs is then used drive a beacondevice, the emission of which can be measured. Timing information isextracted by measuring photon counts (photon detector output) which islinearly proportional to I_(emission). Thus, as shown in FIG. 4, inaccordance with an embodiment of the present invention, the PFC 11 isfollowed by a modulator 12 which drives the beacon transistor 13. Nowthe output pulse, which has an amplitude proportional to emissionintensity, provides a measure of timing difference Δt.

[0018] Thus, in accordance with embodiments of the present invention, totranslate skew timing information into a Vgs voltage level, a voltagemodulator circuit is required. The circuit will be incorporated into thedie under test. However, an embodiment of such a circuit according tothe present invention, has small area and loading and does not disturbthe operation of the circuit being probed. As described in theaforementioned publications, backside emission from the die is measured.Since embodiments of the present invention measure only the lightemitted from the device, this method is completely non-invasive and willscale well for smaller device geometries (below 100 nm).

[0019] An embodiment of such a circuit is shown in FIG. 5. It includes aseries circuit extending between Vcc and ground made up of an NMOSprecharge device PPRE 51, two qualifying devices, a PMOS device 53 andan NMOS device 55. The junction between device 51 and device 53 iscoupled to one terminal of a capacitor NCAP 57 which has its otherterminal coupled to ground. The gate of device 51 is driven by a“precharge” signal. Each of devices 53 and 55 is driven by an switchingdevice. Device 53 is driven by an switching device 60 made up of PMOSdevice 59 and NMOS device 61. Device 55 is driven by an switching device64 made up of PMOS device 63 and NMOS device 65. Switching device 60 isdriven by a signal “CLKEarly” and switching device 64 by a signal“CLKLate.” The are the two signal, the delay or skew between which is tobe measured. The gate of the beacon transistor 10 is coupled the gate ofNCAP 57 and the junction between the drain of device 51 and the sourceof device 53.

[0020]FIG. 6 shows the timing waveforms for the embodiment of thecircuit of FIG. 5. FIGS. 7a-c repeat the modulator portion of FIG. 5 andare helpful, along with FIG. 6 in understanding the operation of thecircuit. FIG. 7a is the modulator circuit by itself. In operation,first, the capacitor 57 is precharged to (Vcc−Vtn) through device 51,when CLKEarly is low. This is illustrated in FIG. 7b, where the darklines show the part of the circuit active during precharge. At thispoint, as FIG. 3 suggests, the pre-charged Vgs line (=Vcc−Vtn) sets theemission intensity to a maximum.

[0021] Device 55 turns on when the CLKLate signal is low. As theCLKEarly signal goes high, device 53 switches ON, developing aconductive path between the capacitor and ground. The active parts ofthe circuit now are shown in dark lines in FIG. 7c. Now the capacitorbegins discharging the gate voltage, and with it the intensitydecreases. The conduction lasts until CLKLate goes high. The differencebetween the time when CLKEarly goes high and the time when CLKLate goeshigh is the time difference or skew to be measured. Thus, it is apparentthat the longer this time, the more the discharge and the lower theintensity. Thus, effectively the modulator converts a time difference,such as a skew into a value of voltage Vgs. As is explained in moredetailed below, this voltage is then used, in combination with the knownrelationship between Vgs and intensity plotted in FIG. 3, to obtain alinear relationship between intensity, which can be measured asindicated in FIG. 4, and timing difference or skew.

[0022] As illustrated by the embodiment of FIG. 4, it is possible toalso use a Phase-Frequency Comparator (PFC) to focus the mode ofoperation on one particular edge (i.e. rising edge). In this case thetwo outputs of PFC (UP and DOWN) replaces CLKEarly and CLKLate signalsas indicated on FIG. 6.

[0023] Since the qualifying devices 53 and 55 are operating insaturation, the current (Ids) is proportional to the square of thecapacitor voltage.

I _(ds)=β₁(V _(gs1)/2)²(1+λ.V _(dS1))=β₂(V _(gs2)/2)²(1+λ.V_(ds2))  (equation 1)

[0024] For very small number of λ, Ids α V_(gS1) ² α V_(gs2) ²

[0025] where the subscript 1 represents device 53 of FIG. 5 andsubscript 2 represents device 55 in the same figure.

[0026] Thus,

R _(dS1) =V _(gs1) /I _(ds1) αV _(gs1) /V _(gs1) ²=1/V _(gs1)  (equation2)

[0027] Likewise R_(ds2) α 1/V_(gs2)

[0028] The exponential behavior of I_(emission) vs. V_(gs) representedby circle points in FIG. 8 is compensated by the inverse relationshipbetween V_(gs) and R_(ds) shown in FIG. 2. With R_(ds) linearlyproportional to skew, the linear relationship between I_(emission) andskew is guaranteed. These relationships are evident from the graphs ofFIGS. 8 and 9a-9 c. FIG. 9a shows the relationship between R_(ds) andclock skew. The linear relationship is apparent. FIG. 9b shows therelationship between Ids and clock skew. Finally, FIG. 9c shows therelationship between the Vgs voltage and clock skew. Also shown is thevoltage at nodes n0, n1 and n2 of FIG. 5.

[0029] Turning to FIG. 8, three curves are plotted. First, timingdifference or skew vs. gate voltage is plotted as curve 101. This is thesame relationship as shown in FIG. 7c. Intensity vs. gate voltage isplotted as curve 103. This corresponds to curve 21 of FIG. 3. Fromthese, the relationship of intensity vs. skew is obtained and plotted ascurve 105. Again, the linearity is apparent. The non-linearity of theskew vs. gate voltage has compensated for the non-linearity in theintensity vs. gate voltage relationship. It can be seen that with notiming difference measured, Vgs and intensity are at their normalizedvalue of 1. As time increases, and the capacitor 57 of FIG. 5discharges, Vgs, and with it intensity decreases. The larger the timedifference, the more the decrease as shown by curve 105 of FIG. 8. Thus,with embodiments of the modulator of the present invention, it ispossible to determine the timing difference or skew by means ofmeasuring the intensity of the emission from the beacon transistor.

[0030] In summary, the voltage modulator according to embodiments of thepresent invention has two functions:

[0031] 1. to convert a time difference which may be skew timinginformation into a Vgs voltage level controlling a beacon device; and

[0032] 2. to linearize the relationship between I_(emission) responseand skew. By doing this, the ‘deadband region’ of the prior art isavoided. Furthermore, there is no perturbation to clock operation due tosmall area and loading, a linear relationship between I_(emission) andskew is maintained avoiding distortion due to non-linearity. As notedabove embodiments of the present invention also scale well for smallerdevice geometries (below 100 nm).

[0033] Embodiments of a method and apparatus to translate timingdifferences into emission intensity have been described. In theforegoing description, for purposes of explanation, numerous specificdetails are set forth to provide a thorough understanding of the presentinvention. It will be appreciated, however, by one skilled in the artthat the present invention may be practiced without these specificdetails. In other instances, structures and devices are shown in blockdiagram form. Furthermore, one skilled in the art can readily appreciatethat the specific sequences in which methods are presented and performedare illustrative and it is contemplated that the sequences can be variedand still remain within the spirit and scope of the present invention.

[0034] In the foregoing detailed description, apparatus and methods inaccordance with embodiments of the present invention have been describedwith reference to specific exemplary embodiments. Accordingly, thepresent specification and figures are to be regarded as illustrativerather than restrictive.

What is claimed is:
 1. A non-invasive method to accurately measure thetime difference between two signals comprising detecting the emissionfrom a beacon device the intensity of which is modulated as a functionof said time difference.
 2. A non-invasive method to accurately measurethe time difference between two signals according to claim 1 whereinsaid time difference is a time difference between two timing signals. 3.A non-invasive method to accurately measure the time difference betweentwo signals according to claim 1 wherein said time difference is a clockskew.
 4. A non-invasive method to accurately measure the time differencebetween two signals according to claim 1 comprising: a. converting saidtime difference into a voltage level; b. supplying said voltage to abeacon device having an emission response; and c. linearizing therelationship between said emission response and said time difference. 5.A non-invasive method to accurately measure the time difference betweentwo signals according to claim 4 and further comprising detecting theintensity of light emitted from said beacon device.
 6. A non-invasivemethod to accurately measure the time difference between two signalsaccording to claim 5 wherein said step of converting comprises: a.precharging a capacitor with a supply voltage; and b. discharging saidcapacitor for a period equal to the time difference between two signals.7. A non-invasive method to accurately measure the time differencebetween two signals according to claim 6 wherein the relationshipbetween said voltage and said time difference in non-linear and saidstep of linearizing comprises providing a beacon device having acharacteristic relationship between voltage and intensity whichcompensates for said non-linear relationship.
 8. A non-invasive methodto accurately measure the time difference between two signals accordingto claim 7 wherein said beacon device is a saturated NMOS transistor. 9.Non-invasive apparatus to accurately measure the time difference betweentwo signals having edges delayed with respect to one another comprising:a. a voltage modulator circuit receiving the two signals as inputs andproviding an output voltage modulated with said timing difference; b. abeacon device receiving the output voltage of said modulator circuit andconverting it into an emission intensity.
 10. Non-invasive apparatus toaccurately measure the time difference between two signals according toclaim 5 wherein said time difference is a time difference between twotiming signals.
 11. Non-invasive apparatus to accurately measure thetime difference between two signals according to claim 5 wherein saidtime difference is a clock skew.
 12. Non-invasive apparatus toaccurately measure the time difference between two signals according toclaim 5 wherein said step voltage modulator circuit comprises: a. acapacitor from which said modulated output is supplied; b. a prechargetransistor which when actuated couples said capacitor with a supplyvoltage to precharge it to that voltage; and c. a discharge circuitcoupled to discharge said capacitor for a period equal to said timedifference between two signals.
 13. Non-invasive apparatus to accuratelymeasure the time difference between two signals according to claim 6wherein the relationship between said output voltage and said timedifference is non-linear and said beacon device has a characteristicrelationship between voltage and emission intensity which compensatesfor said non-linear relationship.
 14. Non-invasive apparatus toaccurately measure the time difference between two signals according toclaim 7 wherein said beacon device comprises a saturated NMOStransistor.
 15. Non-invasive apparatus to accurately measure the timedifference between two signals according to claim 7 wherein saidprecharge transistor comprises an NMOS transistor having a sourcecoupled to the supply voltage, a drain coupled to a terminal of saidcapacitor and a gate, the capacitor having another terminal coupled toground.
 16. Non-invasive apparatus to accurately measure the timedifference between two signals according to claim 7 wherein saiddischarge circuit comprises a series circuit made up two qualifyingdevices, a PMOS device and an NMOS device, the PMOS device driven by afirst of said two signal and the NMOS device driven by a second of saidtwo signals, said PMOS device coupled to one terminal of said capacitor.17. Non-invasive apparatus to accurately measure the time differencebetween two signals according to claim 16 wherein said first signal is a“CLKEarly” signal and said second signal is a “CLKLate” signal. 18.Non-invasive apparatus to accurately measure the time difference betweentwo signals according to claim 16 and further including first and secondswitching devices respectively between said PMOS device and said NMOSdevice and said first and second signals.
 19. Apparatus to accuratelymeasure the time difference between two signals comprising: a. a beacondevice having an emission response; b. means to modulate the intensityof the beacon device as a function of the time difference; and c. meansto detect an emission from the beam device.
 20. Apparatus to accuratelymeasure the time difference between two signals according to claim 19wherein said means to modulate comprise: a. means to convert said timedifference into a voltage level; b. means to supply said voltage to thebeacon device; and c. means to linearize the relationship between saidemission response and said time difference.
 21. Apparatus to accuratelymeasure the time difference between two signals according to claim 20wherein said means to convert comprise: a. means to precharge acapacitor with a supply voltage; and b. means to discharging saidcapacitor for a period equal to the time difference between two signals.22. Apparatus to accurately measure the time difference between twosignals according to claim 20 wherein the relationship between saidvoltage and said time difference is non-linear and means to linearizecomprise a beacon device having a characteristic relationship betweenvoltage and intensity which compensates for said non-linearrelationship.
 23. Apparatus to accurately measure the time differencebetween two signals according to claim 22 wherein said beacon device ina saturated NMOS transistor.